Joining Techniques Critical to Auto Composites

We've addressed some of the many manufacturability challenges carbon composites face in automotive production. But other technical problems must also be solved before carbon is ready for mass manufacturing. One of the biggest is the lack of standards and technologies for joining techniques.

Carmakers have optimized processes for metals. But Peter Cate, global strategic marketing manager for composites at Dow Automotive Systems, told us carbon fiber and epoxy resin are radically different materials, and so are their joining techniques, since they can't be welded.

"You need highly specialized structural adhesives, so this affects the joining and assembly to build the vehicle," he said. "More pieces of the puzzle include combining fiber and resin, how to join composite components together, and how to join them with metal."

Automating gluing techniques for joining composites will require high speed and high accuracy. ABB is integrating dispensing controls into its robots so their speed, path, and glue dispensing are controlled by the robot controller. This robot demonstrates the same quality of glue bead whetherthe dispensing speed is 100mm/sec or 1,000mm/sec.(Source: ABB Robotics)

Jay Baron, director of the Coalition of Automotive Lightweighting Materials (CALM) and president of the Center for Automotive Research (CAR), told us joining technologies for carbon fiber are a challenge. "You must use hard attachment or adhesives," he said. "Adhesives aren't standard across suppliers and require sophisticated application devices like robots. Also, the repairability of adhesives, as well as predicting their performance, is difficult."

The Airbus A380 aluminum/composite wing problem (which we discussed here) demonstrated the major challenge of structural joining techniques for mixed materials. Srinivas Nidamarthi, ABB Robotics' technology manager for automotive robotic systems, told us the growing use of plastic and composites brings similar challenges to automotive manufacturing. "One common challenge in structural components is how to combine and join mixed materials and still preserve structural strength, or crashworthiness, so OEMs are working on joining technologies."

Robotic assembly techniques are being investigated to speed up composite production. Nidamarthi said robotic systems could also be used in post-production tasks such as joining, coating, and painting. "We're working on automating those joining technologies, such as hemming, clinching, and rivetting, to make them mass-produceable," he said. "Hemming and clinching require bending or deforming a material. This works as long as you can elastically bend a material to make a joint." But different materials have different elasticity, and metal deforms differently than plastics, so these joining techniques are limited by the behavior of the materials.

Fascinating story, Ann, on how to join materials that can't be welded. One thing that gives me pause is that as they develop appropriate adhesives, it will still only be a guess on how the joining materials will hold up after 20 years. Only time will tell whether the adhesives will hold the car together over decades.

These are structural adhesives, and many of them are being adapted from aerospace applications, where they've been used with composites for some time. We covered structural adhesives herehttp://www.designnews.com/document.asp?doc_id=237011

I did some work at Raytheon (Beech) quite a number of years back. They routinely used adhesives to bond aluminum aircraft parts together. The parts were also riveted (sparingly), but the engineers referred to the rivets as "chicken rivets", because they insisted the glue was more than strong enough. Supposedly the FAA wouldn't let them get rid of all of the rivets. I don't know how it ages, however. We had to disassemble some of the parts that were glued, and the aluminum would tear before the glue would let go. As I remember it, the stuff was basically inseparable.

That's very encouraging, Ttemple. Was that a number of years ago? I would guess if adhesives had any durability problems we would know by now. In manufacturing, I would think using adhesives is more efficient than welding.

I would say around 1996 or so. They were manufacturing the (Kingair?)"1900D" heavily at that time. It says on wikipedia that the 1900D was introduced in 1991, and produced through 2002. I'm sure that many 1900D's are still in use as regional commuters.

I don't know when they started using adhesives though. The part I remember specifically was the window attachment to the fuselage skin. There was sort of an aluminum porthole looking piece that was glued to the fuselage. The joint had the appearance of a weld, and it held together like it.

I think they use adhesives in the wing compartments too, at least in the areas where there is going to be fuel stored. The adhesives seal the compartments, I think. They also use special fuel resistant coatings on the skin inside those areas.

ttemple, thanks for telling us about your direct experience. Anything going into the construction of commercial aircraft has very strict specifications and requirements, including extensive testing on the ground and in the air, and everything is 100% traceable. Whether structural adhesives or fasteners are used in a particular part of the plane depends on several factors, but stresses in aircraft are much more extreme than in cars.

Ttemple, it sounds like there is an good long track record on the adhesives, at least a couple decades. So, while these adhesives may be new to the auto industry, they seem to be well tested in aerospace.

It certainly is a valid question, adhesive lifetime, and how does one speed the aging process so as to find a correct answer? And the very important question is how reliableare the bond lifetime results? Many adhesives primarily fail through long term creep type of fault, while in others t6he ridgid bond becomes brittle and does not stand up to shocks. Two different failure mechanisms, it seems. And he experience of how things stick to a composit is not the level that we are looking for. So the solution is to understand the failure mechanism, and use that information.

A lot of research has been done on structural adhesives, as we've covered here http://www.designnews.com/document.asp?doc_id=237011 and here http://www.designnews.com/document.asp?doc_id=236816 Fabrico, which doesn't make adhesives, but uses them assembling all kinds of structural components, has articles and a Q&A on many of these subjects here http://fabricoforum.com/2012/06/structural-adhesives/

University of Southampton researchers have come up with a way to 3D print transparent optical fibers like those used in fiber-optic telecommunications cables, potentially boosting frequency and reducing loss.

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